When XDEBUG is enabled, check for SelectionDAG cycles at some key
points.  This will help us find future problems like the one
described in PR6019.

llvm-svn: 94019
diff --git a/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp b/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
index 13f4030..01cad71 100644
--- a/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
+++ b/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
@@ -598,6 +598,7 @@
     if (RModW) {
       MoveBelowTokenFactor(CurDAG, Load, SDValue(I, 0), Chain);
       ++NumLoadMoved;
+      checkForCycles(I);
     }
   }
 }